Mass Movement

Slope forms: crest, free face, talus and pediment.Mass MovementCreepSlumpDebris flowEarth flowRockslidesHuman Land use and LandslidesPreventing Landslides

y Mass Movement

Slopes

Figure 9.3

Crest Convex slope Initial Upland Weathering and soil creep Weathered material falls over the cliff

Free Face Steep slope with active erosion Bare rock face Rainwater channelled into gullies called rills Erosion, slumping and weathering cause the cliff face to retreat

Talus Slope Accumulation of debris from free face Slope angel of 35-37 degrees Large boulders near cliff face, pebbles and stones further away result in a

more gentle angle Small particles called FINES washed out onto the pediment

Pediment Gentle slope of half to 5 degrees Concave slope Covered by fines from the Talus slope Unconsolidated material is easily washed away.

Talus Slope at Wheeler Peak, Great Basin National Park, Nevada.

Accumulation of debris from free face.Slope angle of 35-37 degrees.Large boulders near cliff face, pebblesand stones further away result in a more gentle angle.Small particles called fines are.washed out onto the pediment.

Source: Tom Bean/DRK Photos

Identify the following slopeand list 4 characteristics of this slope in a textbox below.

Fig. 11-8b, p. 253

Identify this slope and list 4 characteristics of the slope.

Free Face :Steep slope with active erosion.

Bare rock face.

Rainwater channelled into gullies called rills.

Erosion, weathering and slumping cause the cliff face to retreat.

esSlope forms: crest, free face, talus and pediment.Mass MovementCreepSlumpDebris flowEarth flowRockslidesHuman Land use and LandslidesPreventing Landslides

Mass MovementG

Soil CreepSolifluctionEarth and Mud flowsLandslides and Rockfalls

Source: Roger J. Wyan/AP Wide World Photo

Mass Movement

Mass Movement1 Mass wasting is the downslope movement of Earth material

due to gravity. Factors such as slope angle, weathering and climate, water content, vegetation, and overloading are interrelated, and all affect mass wasting.

2 Mass movements can be triggered by such factors as soil saturation and ground shaking.

3 Mass wasting can be categorized as resulting from either rapid mass movements or slow mass movements.

4 The different types of rapid mass movements are rockfalls, rock slides, mudflows, debris flows, and some slumps; each type has recognizable characteristics.

5 The different types of slow mass movements are earthflows, solifluction, soil creep, and some slumps; each type has recognizable characteristics.

6 People can minimize the effects of mass wasting by conducting geologic investigations of an area and stabilizing slopes to prevent and ameliorate movement.

Table 11-2, p. 252

Important types of mass wasting

• Slide – downslope movement of coherent block of earth material

• Slump – is sliding along a curved slip plane producing slump blocks

• Fall – rocks fall from vertical face• Flow – Downslope movement of unconsolidated material in

which particles move about and mix within the mass• Subsidence is the sinking of of a mass of earth material below

the level of surrounding material

Factors Affecting Slope Stability

• Type of earth material• Slope Angle and Topography• Climate• Vegetation• Water• Time

Triggers for rapid Mass Wasting

• Rain• Oversteepening

– cutting at foot of slope– piling on head of slope

• Deforesting / Devegetating• Earthquakes

Some Common Processes that Oversteepen Slopes

Some Common Processes that Oversteepen Slopes (cont’d)

Some Common Processesthat Oversteepen Slopes (cont'd)

Slope forms: crest, free face, talus and pediment.Mass MovementCreepSlumpDebris flowEarth flowRockslidesHuman Land use and LandslidesPreventing Landslides

Mass MovementG

Creep• very slow downslope movement of soil• result of freezing and thawing

from D. Schwert, NDSU

Fig. 11-5a, p. 249

Fig. 11-5c, p. 249

Fig. 11-5d, p. 249

Fig. 11-6, p. 249

Fig. 11-2, p. 247

esSlope forms: crest, free face, talus and pediment.Mass MovementCreepSlumpDebris flowWaters role in mass movementEarth flowRockslidesHuman Land use and LandslidesPreventing Landslides

Mass MovementG

A slump is a type of slope failure in which a downward and outward rotational movement of rock or regolith occurs along a curved concave-up surface. – Often the result of artificial modification of the

landscape. – Associated with heavy rains or sudden shocks,

such as earthquakes.

Slump (a type of slide)• Indicators:

– Scarp– “Hummocky”

terrain on and below (earthflow)

Slope forms: crest, free face, talus and pediment.Mass MovementCreepSlumpDebris / Mud flowEarth flowRockslidesHuman Land use and LandslidesPreventing Landslides

y Mass Movement

Debris Flow• Debris flows are the downslope flow of relatively coarse

material• > 50% of particles in a debris flow are coarser than sand• Movement may be very slow or very fast, depending on

topographic conditions• Contain less water and larger particles than Mudflows.

Turtle MountainTurtle Mountain Rock Fall

Fig. 11-7a, p. 251

Water's Role In Mass Movement

Fig. 11-7b, p. 251

Water's Role In Mass Movement

Water's Role In Mass Movement: Before Slide

Water's Role In Mass Movement: After Slide

Parallel Planes: Mass-Movement Potential

Mass MovementG

Earthflow

Mass Movement

Rockslide• Rock moves because there’s nothing holding it back!• Generally requires a pre-existing low-friction surface...

Rockslide• like a clay layer, once it’s wet...

Fig. 11-8a, p. 253

Fig. 11-8b, p. 253

Mass Movement

Human Land Use and Landslide• Urbanization, irrigation• Timber harvesting in weak,

relatively unstable areas• Artificial fillings of loose

materials• Artificial modification

of landscape• Dam construction

Mass MovementG

Mitigating the Landslide Hazard (2)

• Preventing landslides– Drainage control: Reducing infiltration and surface

runoff– Slope grading: Reducing the overall slope– Slope supports: Retaining walls or deep supporting

piles• Avoid landslide hazards

– Landslide warning for critical evacuations– Correcting landslides

Fig. 11-23a, p. 265

Fig. 11-23b, p. 265

Fig. 11-24b, p. 265

Identify this type of Mass Wasting and explain how it occurs.

A slump is a type of slope failure in which a downward and outward rotational movement of rock or regolith occurs along a curved concave-up surface. – Often the result of artificial modification of the

landscape. – Associated with heavy rains or sudden shocks, such as

earthquakes.

What can be done to minimize damage from Rockfalls?

Use the photographs to assist you in your answer.

Minimizing damage from rockfalls. (a) Wire mesh covers this steep slope in Hawaii. This is a common practice in mountainous areas to prevent rocks from falling on the road. (b) A wire mesh fence along the base of this hillside of Highway 44 in California has caught many boulders and prevented them from rolling onto the highway. But note that some boulders have made it over the fence.

• Creep is extremely slow downward movement of dry surfacial matter.

• Movement of the soil occurs in regions which are subjected to freeze-thaw conditions. The freeze lifts the particles of soil and rocks and when there is a thaw, the particles are set back down, but not in the same place as before.

Identify this type of mass wasting and explain what causes it to occur!

Identify the following types of Mass wastingand tabulate the causes of each.

Rockslide Occurs where nothing is holding the rocks back.Generally requires a pre-existing low-friction surface, like a clay layer once its wet.

Landslide

Caused by:Erosion causing extremely steep slopesA powerful earthquakeExcess weight on unstable soilA Volcanic eruption.

1) Identify this type of mass wasting

2) What must you do to prevent this type of mass wasting?

MUDFLOW

• Plant ground cover on slopes and build retaining walls.

• In mudflow areas, build channels or deflection walls to direct the flow around buildings.

• Install flexible pipe fittings to avoid gas or water leaks.

1) Identify this type of mass wasting

2) Where does this type of mass mostly occur?

Solifluction

• the slow downslope movement of water-saturated sediment

• most common in areas of permafrost.

Schuters pg 204 and 205

Mass Movement

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